1. Field of the Invention
This invention relates to an apparatus and process for locating gas leaks in underground pipelines, and more particularly to sound detection apparatus for identifying and measuring sound waves created by leaking gas. The apparatus and process uses a probe-accelerometer combination having an effective mechanical resonant frequency less than or equal to the electrical resonant frequency of the attached accelerometer. This combination increases the sensitivity of the system. In a preferred embodiment, acoustic shielding is used around the portion of the probe which is above ground and the accelerometer, to reduce system response to airborne noise.
Underground natural gas piplines develop leaks which must be identified, located and repaired. Even relatively small leaks, 1/16 inch or less in diameter, may be hazardous, wasteful and expensive, and must be eliminated. Leaks are usually discovered by using a flame ionization detector and pinpointed by taking a series of measurements along a pipeline with a combustible gas indicator. Both of these instruments measure chemical properties of the natural gas. These methods do not always pinpoint the leak resulting in the first excavation not being at the site of the leak, resulting in added time and expense of further excavating. There is a need for more precise leak detection.
The escaping gas from leaks generally create sound waves which may be detected by ground surface sonic detection equipment, but such equipment must be extremely sensitive to respond to the leak sounds. Leaks can be discovered and pinpointed by taking a series of sound measurements along a pipeline with a transducer or the like. The approximate location of a leak is generally beneath the ground where the measured signal is the strongest. The ground in the vicinity of the strongest signal is excavated until the leak is found. Since the size of excavation is closely related to the precision with which the signal is measured and the leak located, it is desirable to accurately locate leaks, as well as to identify them. Thus, there is a need for improved sensitivity in sound detection equipment which identifies and locates gas leaks in underground pipelines.
When the sensitivity of the sound detection equipment is increased, extraneous noise more adversely affects the ability of the equipment to isolate the sound waves generated by leaks. Airborne noise created by vehicular traffic, industrial activity, weather phenomena, and the like, must be attenuated well below the intensity of the sound waves created by the leaks, or eliminated by electronic circuitry.
2. Description of the Prior Art
Sonic leak detectors and leak pinpointers can be divided into two general groups: detectors for leaks of liquid materials and detectors for leaks of gaseous materials. Liquid leaks generally create louder sound waves than gas leaks and high pressure leaks are generally louder than lower pressure leaks.
Previous efforts to improve the sensitivity of sonic detectors and control extraneous noise have not been as successful as desired, particularly for gas leaks. For example, U.S. Pat. No. 4,176,543 teaches electronic gating circuitry for leak detectors in liquid storage tanks to reduce the effect of noise on the detection system. The circuitry identifies noise pulses received in a transducer and eliminates them from the output signal of the detection circuitry. There is no attenuation of the noise before it is detected in the transducer, however, and the sensitivity of the transducer itself is not improved.
U.S. Pat. No. 4,083,229 teaches location of a leak in an underground water pipe by use of spaced detectors in contact with the pipe, such as secured to fire hydrants, stand pipes or the like to improve sensitivity.
U.S. Pat. No. 3,223,194 teaches a sonic vibration locator for detecting leaks in water pipes. The detector has multiple vibration transmitting prongs in a special shaped base and in special arrangement relative to a transducer. The '194 patent teaches four long probes may be used for detecting vibrations in soft top earth. There are no teachings relative to the length or resonant frequencies of the long prongs.
U.S. Pat. No. 3,055,209 teaches an active sonic leak detector for locating leaks in underground gas conduits by transmitting sonic waves through the gas within the conduit and through the leak opening for detection.
U.S. Pat. No. 3,264,864 teaches a sonic leak detector for locating leaks in underground gas conduits wherein a sonic wave of one sonic frequency is transmitted through the gas conduit and a receiving transducer sensitive to a mechanical wave of a sonic frequency transmitted by the gas through the leak. The '864 patent also teaches that by coupling a pair of such transducers, the need for transmission of a sonic signal in the gas may be avoided and the difference in phase relationship of the natural sounds of the leak are used. The '864 patent teaches a transducer having 3 to 6 prongs for contact with the earth. The patent teaches 1 or 2 prongs do not give satisfactory performance and that very long prongs are not desirable, about 5 inches being most satisfactory.